1. Field of the Invention
The present invention generally relates to a driving system and more particularly, to an apparatus for driving a tracking mirror for moving a light spot with respect to recording tracks on a disk.
2. Description of the Related Art
Recently reducing the weight of the movable portion of an optical head has been made the subject of a very important study on how to attain high speed access to an optical disk.
The above study has deemed a system in which tracking control is effected by a rotary type mirror (galvano-mirror) to be most effective in facilitating high speed access to the disk.
Specifically, such a system does not employ a tracking driving device conventionally mounted on the movable portion of the optical head, whereby the weight of the optical head is comparatively less.
With respect to the above, the following tracking control systems have been proposed.
(1) U.S. Pat. No. 4,330,880 to Ate Van Dijk (May 18, 1982) entitled "Apparatus for optically reading a disc-shaped record carrier in particular for rapidly locating a desired program section" discloses that high speed access can be attained by tracking effected by a galvano-mirror. PA1 (2) U.S. Pat. No. 4,423,496 to Willem G. Opheij et al. (Dec. 27, 1983) entitled "Apparatus for reading and/or writing an optically readable information structure" discloses that, by detecting the amount of rotation of a mirror through the use of a photo-reflector, and using a signal from the photo-reflector as a correction signal for position control, a more stable tracking control servo may be realized. PA1 (1) U.S. Pat. No. 4,466,088 to Paul M. Trethewey et al. (Aug. 14, 1984) entitled "Galvo position sensor for track selection in optical data disk system" discloses an arrangement in which a second auxiliary reflecting mirror is provided adjacent the reflecting side of a galvano-mirror, and light reflected from the second reflecting mirror, the reflecting side of the galvano-mirror, and then light from the second reflecting mirror is monitored to achieve higher sensitivity.
Additionally, because the accuracy of the control depends on the accuracy of the detection of the amount the mirror has rotated, various mirror displacement detecting apparatuses employing light have been proposed, for example, as follows.
Subsequently, one example of conventional mirror driving apparatuses will be described.
In FIGS. 8(a), 8(b), 8(c), and FIGS. 9(a), 9(b), the known mirror driving apparatus generally includes a rotary mirror 1, a plate or leaf spring 2 to one surface of which the rotary mirror 1 is bonded, and a driving coil 3 which is bonded to the other surface of the plate spring 2. A winding axis of the coil 3 extends vertically. A rotary section 4 is thus constituted by the rotary mirror 1, plate spring 2 and driving coil 3. Opposite end portions of the plate spring 2, disposed along a rotary axis 6 of the rotary section 4, are bonded to a housing 5 so that the rotary section 4 may be rotated in directions indicated by arrow 9. A permanent magnet 7 is fixed in the housing 5 within an opening of the driving coil 3, and is magnetized in a direction 8 perpendicular to the rotary axis 6 of the rotary section 4 and parallel to a mirror surface of the rotary mirror 1.
In FIGS. 10(a) and 10(b), a tracking control signal 31 is applied to a driving circuit 10 which drives the driving coil 3 at a current value proportional to a voltage value of the tracking control signal 31. An objective lens 12 converges laser light 13 reflected by the rotary mirror 1 to form a light spot 14 on the track of an optical disk 11. The tracking direction is denoted by arrow 15. The driving circuit 10 may be a voltage-current converter which drives the driving coil 3 with current proportional to the tracking control signal, and therefore, a detailed description thereof is omitted here for the sake of brevity.
The operation of the mirror driving apparatus will be described hereinbelow.
In the first place, for causing the light spot 14 to follow the tracks on the optical disk 11, the tracking control signal 31 is applied to the driving circuit 10 of the mirror driving apparatus as a target driving signal of the tracking mirror driving apparatus. Subsequently, a driving signal corresponding to the tracking control signal 31 is passed through the driving coil 3, which is subjected to an electromagnetic force corresponding to the tracking control signal 31 from the permanent magnet 7. As a result, the rotary section 4 constituted by the rotary mirror 1, the plate spring 2, and the driving coil 3 is rotated in direction 9 about the rotary axis 6. Accordingly, the angle at which the laser light 13 is reflected by the rotary mirror 1 is altered. Consequently, the angle of incidence of the laser light 13 on the objective lens 12 is altered, thus displacing the light spot 14 in the tracking direction 15 of the optical disk 11 (FIG. 10(a)).
By the above function, tracking control in the optical disk apparatus can be realized.
However, the conventional mirror driving apparatus has a problem due to the fact that a mechanical vibrating system, in which the rotary section 4 is supported at opposite ends of the plate spring 2, is used. Specifically, the resonance of the rotary section 4 cannot be suppressed, and thus, stable tracking cannot be ensured.
Meanwhile, altering the value of the initial resonance frequency is often carried out by adding a mechanical damper, for example, by sticking damper members to the rotary portion of the plate spring, etc. Although the resonance may be suppressed by such a practice, there is a drawback in that the rotary section 4 cannot be rapidly restored to its initial position, with a consequent increase of a dead band region (hysteresis characteristic) of the rotary mirror 1, thus making it impossible to maintain the initial position of the rotary mirror 1.